Many electronic devices require the placement of conductive lines on a substrate, such as in the case of printed circuit boards, hybrid thick-film components, and the like. These conductive lines serve as electrical interconnections between discrete components, between components and interconnection means, or as conduits between electrical connectors, for example. It has been a continuing trend in the electronics industry to miniaturize the components, connectors, and other devices, so as to provide greater performance, efficiency and portability of electronic apparatus. One factor limiting the extent of miniaturization has been the physical space required for the interconnecting conductive lines themselves.
In the particular industry relating to thermal printing aparatus, miniaturization of components and circuitry is one approach to the development of thermal printers which are capable of exceptionally high print resolutions. In a thermal printer, heat-sensitive paper, tape, or other thermal medium, is placed in contact with a heating element; when the heating element is electrically energized, the transmitted heat causes a mark to appear on the paper, at the location of the heating element and in the shape of that heating element. A row of heating elements can thus simultaneously print a complete row of marks, such as dots, for example. If the heat-sensitive paper is moved along a row of dot-shaped heating elements at a predetermined speed, the heating elements can be energized and de-energized in such a fashion in timed correspondence with the paper movement so that recognizable patterns of dots are formed on the paper. These patterns may be letters, numbers, other characters, designs, or virtually any desired image. The part of thermal printing apparatus which produces the heated areas is known as a thermal print head.
The thermal print head comprises a row of heating elements (either discrete or continuously formed) mounted on a substrate or base, with controlling circuitry for determining operation of the individual heating elements, means for attaching the print head to the rest of the thermal printing apparatus, and electrical interconnection means for providing the control circuitry and heating elements with power, ground connection, and data signals.
In one particular type of print head, the individual heating elements are formed by adjacent contiguous sections of a continuous resistor bar, customarily known as an R-bar. Directly underneath the R-bar are interleaved conductive fingers, with every other finger being a ground finger. All of the ground fingers are connected together by a common ground conductor line adjacent to and extending along the length of the R-bar. Interleaved between each pair of ground fingers is a signal finger, connected by individual conductive lines printed on the surface of the print head substrate or base material to the controlling circuitry, located away from the R-bar, to avoid interfering with contact of the moving heat-sensitive paper with the R-bar. In order to continue the trend of miniaturization of the print head, improved methods of placing these conductive finger lines on a substrate more closely and with thinner lines had to be developed.
There are at least three ways to place conductive lines on a substrate: chemical etching; ion, or plasma, beam etching, and screen-printing. Chemical etching involves the use of a mask, resembling the desired pattern of conductive lines, which is placed over a film of conductive material. The exposed conductive material is then chemically etched away at the openings in the mask, thus leaving the desired conductive pattern on the substrate. The chemical etching process, however, has several inherent disadvantages. Many different process steps and materials are required, including application of a conductive film to a substrate, application of a mask onto the conductive film, etching in a chemical bath, stopping the chemical etching reaction, cleaning the workpiece, and so on. In addition, each of the process steps is highly sensitive to slight variations in operating parameters, such as temperature of the solutions, uniformity of the conductive film and mask materials, and especially, timing of the etching step. Furthermore, the chemicals and processes employed are potentially hazardous to operators and technicians, and require great care in handling. These factors combine to make chemical etching an expensive and variable method of applying conductive lines to a substrate.
An additional problem with chemical etching arises it is desired that very fine and closely spaced spaced lines remain on the substrate. For any thickness of conductive film, a predetermined time is required for the etchant to completely remove the film at a particular location. The action of the etchant is not unidirectional, however, and during the time required for the etchant to work completely down through a conductive layer, the etchant also etches laterally, under the mask. Because of this "undercut", the remaining conductive lines are narrower at the top of the lines than at the bottom surface adjacent to the substrate. Additionally, corners which were sharp on the mask become rounded, to a certain degree. These problems add to the expense and variability of chemical etching.
Ion, or plasma, beam etching is an emerging technology which utilizes a stream of charged particles to etch away conductive material exposed through openings in a mask, or photo-resist layer placed over the conductive film. This method and its associated apparatus, while it promises to provide unidirectional etching without undercutting, is not sufficiently understood or developed to be commercially practicable on a widely available basis.
Screen printing is a "positive additive" technique, fundamentally different from the two processes mentioned above, in that conductive material is applied to a blank substrate in precisely the desired pattern. This is achieved by the use of apparatus including a screen having apertures corresponding to the desired conductor line pattern, a conductive ink or paste, and a squeegee for forcing the ink through the screen onto the substrate held under the screen. Screen printing in this manner is less expensive, more reliable, and far simpler than either chemical etching or ion beam etching.
Until the present invention, the narrowest conductive lines producible by practical screen printing were approximately 0.005 inch to 0.007 inch (5-7 mils) wide. A major disadvantage with the inks and screens in previous use, was the inability to prevent the ink from spreading laterally after having been squeegeed through the screen. A conductive ink which would not spread after squeegeeing and an appropriate screen for use in conventional screen printing apparatus would provide a simple, inexpensive, widely available method and apparatus for placing closely spaced narrow conductive finger lines on a substrate at higher resolutions than achievable with other available screen printing apparatus.